1. Field of Invention
The present invention relates to a method for preparing organic carbonates, and more particularly, to a method of for preparing organic carbonates from an alcohol compound.
2. Description of Related Art
Organic carbonates find use as a solvent, a lubricant, a plasticizer and a monomer for organic glasses, and have been used in methylation and carbonylation process for preparing isocyanates, polyurethanes and polycarbonates. Furthermore, organic carbonates have been used as anti-explosive additive for gasoline or fuels heavier than gasoline.
Conventionally, organic carbonates are synthesized by phosgenation of methanol (phosgene route). However, phosgene is a toxic chemical and causes corrosion to the reactor, thus phosgene route has been gradually replaced by oxidative carbonylation of an alcohol in the presence of carbon monoxide and oxygen (oxidative carbonylation route) in recent years. Compared with phosgene route, oxidative carbonylation route has advantages such as easy acquirement of starting materials, simple synthetic procedures, less environmental pollution and lower production cost.
Oxidative carbonylation of an alcohol can be performed either in gas phase or in liquid phase. For gas-phase oxidative carbonylation of alcohols, one representative example is disclosed by U.S. Pat. No. 5,162,563, which comprises bringing carbon monoxide into contact with an ester of nitrous acid in a vapor phase in the presence of a solid catalyst, for example, PdCl2 combined with copper.
For liquid-phase oxidative carbonylation of alcohols, various catalysts or catalyst systems have been proposed. For example, EP0460732 discloses using cuprous chloride (CuCl) as catalyst. U.S. Pat. No. 4,218,391 and U.S. Pat. No. 4,318,862 disclose a catalyst comprising a salt of a metal belonging to the Groups IB, IIB and VIIIB of the Periodic Table, preferably the salts of monovalent copper (for example CuCl and CuBr). In the process of these patents, in order to elevate reaction rate, it is necessary to use high concentration of cuprous chloride; however, such high concentration of cuprous chloride may cause corrosion to the reactor. To resolve this problem, the reactor is provided with glass liner on its inner wall. However, the presence of glass liner would enlarge the reactor, which was undesirable from the viewpoint of space utilization.
Chinese Patent CN1197792 discloses a two-component catalyst system for liquid-phase oxidative carbonylation of alcohols, comprising cuprous chloride as catalyst and one inorganic salt such as MgCl2, CaCl2, ZnCl2, KCl etc. as additive. JP No. 54-24827 discloses a similar two-component catalyst system including cuprous oxide as catalyst and a halide of alkali metal or alkaline earth metal as additive. Although these two-component catalyst systems can increase the solubility of CuCl in the reaction medium, they still have the problem of corrosion to the reactor.
U.S. Pat. No. 4,370,275 discloses a catalyst comprising as the essential components, (a) copper and/or copper ions, (b) one or more anions selected from oxide anion, hydroxide anion and carbonate anion, (c) halide ions, and (d) one or more nitrogen bases. A representative catalyst system thereof comprises Cu(II)O, Cu(II)Cl2 and pyridine.
The previous disclosed catalysts are still not satisfactory since the yield of organic carbonates is low and/or corrosion to the reactor may occur. Therefore, a method for preparing organic carbonates, which can increase the reaction rate and yield as well as reduce the problem of corrosion to the reactor, is desired.